1. Field of the Invention
In the past the thermodynamic cycles of automobile engines were optimised for thermal efficiency and specific power.
Nowadays, added to the criteria of efficiency and specific power are the constraints of depollution and particularly the elimination of NOX. These constraints are currently limited to the conditions of urban use of the engine and to road journeys at low power. The foreseeable tightening up of the regulations will lead to an extension of the depolluted range of use of the engine.
The depolluted range is currently limited to 50% of the maximum speed and 50% of the maximum torque, the engine being supplied with fresh air within the range of the raised torques and power.
Numerous techniques for depollution by post-treatment of the gases discharged into the atmosphere are used or are in the course of development, such as oxidoreduction catalysts and regeneratable filters for particles and for NOX.
Amongst the pollutants, the most difficult ones to post-treat in the presence of oxygen are NOX, and attempts are made to eliminate these at source by diluting the fresh air with exhaust gases (EGR) which are recirculated externally or recycled internally. In order to limit the flame temperature sufficiently, the flow rate of EGR by mass must reach 50% of the thermodynamic mass present in the cylinder.
The drawback of this process is to reduce the volume available in the cylinder by 50% in order to collect the fresh air necessary for combustion. Therefore the re-establishment of the power necessitates doubling of the pre-compression of the combustive charge by turbocharging.
Moreover, a turbocharged automobile engine must supply the breakaway torque of the vehicle at the clutch speed and its maximum torque at a speed which is as low as possible. The supercharging pressure must therefore be established very quickly when the engine passes from the idling speed to the clutch speed.
2. Description of Related Art
The industry is researching turbomachinery capable of delivering a variable volume of air at a constant air pressure of approximately 2.5 bars over the entire useful range of speeds (number of revolutions per minute) of the engine which nowadays extends from 1 to 4 approximately.
This level of pressure derives from one single compression stage with a characteristic diagram as wide as possible.
The output section of the turbine must vary substantially within the same proportions as the flow of air.
The solution with the best performance currently is the turbine with a variable distributor which can cover a homogeneous range of 1 to 3 with the maximum width of the compressor field.
At the extremities of this range the efficiency of the compressor is approximately 60% and that of the turbine 50%. These efficiency rates improve towards the centre of the range to reach 75% and 65% respectively.
Therefore a quarter of the engine speed is not covered by the compressor. The torque at low speed is generally favoured and the power decreases from 75% of the maximum speed.
Another solution consists of bypassing the turbine by a controlled valve known as a waste gate. The range of flow rate only goes from 1 to 2. The expansion efficiency decreases between the minimum flow and the maximum flow.
In order to compensate for the dissipation of energy and to extend the flow rate range, it is necessary to increase the exhaust pressure at the expense of an increase in the pumping losses.
These solutions with low energy efficiency are sufficient at moderate pressure where the enthalpy available in the exhaust gases is excessive.
For a double pressure the global turbocharging efficiency must be improved.
The external recycling of cooled gases is managed by a controlled EGR (exhaust gas recirculation) valve which diverts towards the intake a cooled fraction of the gas flow emitted by the engine in the depolluted range exclusively. When this fraction exceeds a limit, the exhaust temperature becomes insufficient to ensure the turbine/compressor energy balance. In order to compensate for this temperature deficiency at depolluted low speeds, the rate of expansion is increased by reducing the section of the turbine at the expense of a decline in the indicated efficiency. When the speed increases, the distributor or the waste gate opens progressively in order to reduce the EGR rate and to limit the back pressure.
This manoeuvre is only possible above a certain speed which depends upon the size of the turbine.
These operations are carried out with poor energy efficiency due to the energy losses in the waste gate or in the variable distributor of the turbine. Furthermore, the back pressure increases the engine consumption over the depolluted range of operation which is very much used in urban driving.
The variable geometry of the turbine is very stressed in urban driving.
In order to improve the expansion efficiency it is necessary to keep to turbines with fixed geometry and to limit the operations with laminations of the flows.
The two-stage compression makes it possible to generate high pressures by taking advantage of the cooling between stages which reduces the compression work.
In order to generate the pressure necessary for the vehicle breakaway torque, the high pressure section of the HP turbine must be sufficiently small to allow expansion of the gas flow emitted by the engine at the clutch speed, that is to say approximately 20% of the volume flow at the maximum speed. In order to limit the exhaust back pressure at high speeds it is necessary to increase the section offered to the gases when the speed increases.
The R2S method of 3K WARNER provides for the mounting in series of the two compressors and the two turbines. In order to increase the section offered to the gases, the gas flow is transferred progressively from the small HP turbine to the large low-pressure LP turbine at the expense of a loss of energy in the regulated bypass of the high-pressure turbine. The increase in the section offered to the gases is limited to the section of the LP turbine. Moreover, the opening of the bypass cancels the rate of expansion of the HP turbine which no longer drives the HP compressor which constitutes a throttling which must be bypassed.
The sequential turbocharging provides for the turbochargers and the turbines to be mounted in parallel. One single turbocharger is active at low speed whilst the two compressors are active at high speed. The transition is made at the much-used intermediate speeds which are used a great deal with a drop in the turbocharging output.
This solution has the advantage of offering to the gases a maximum section equal to the sum of the two turbines.
As before, the transition is made with a loss of energy by lamination in a much-used zone.
Moreover, the air pressure is limited as in the case of the single turbocharger.
In the two preceding cases certain transitions involve the acceleration of one of the rotors, which may prove too slow in the rapid transitions of urban driving.
In order to avoid breaks, the patent application W002/48510 describes a method of unregulated turbocharging with two stages of fixed geometry mounted in series where the pressure in the cylinder is limited by the loss of pressure created by the undersized intake ports. This very simple solution improves the performance at low speed to the detriment of the performance at high speed, where the pumping losses are high, the exhaust pressure being proportional to the speed.
The present invention relates to a method of turbocharging using the advantages of the series and parallel configurations in an original strategy for recycling of the gases.